1. Field of the Invention
The present invention relates generally to an electronic control system for an automatic power transmission for an automative vehicle, for controlling selection of a speed ratio according to a vehicle driving condition. More specifically, the invention relates to a shift control system for the automatic power transmission, which can reduce or suppress shift shock caused by abrupt engine speed and torque variation.
2. Description of the Background Art
Automatic power transmission control systems have been developed in various fashions. For example, "Nissan Full-Range Electronically Controlled Automatic Transmission RE4R01A Service Manual", published by Nissan Motor Co., Ltd., on March 1987, shows an electronically controlled automatic power transmission. Also, similar electronically controlled automatic power transmission has been disclosed in "NISSAN SERVICE MANUAL No. 578" pages C-1 through C-60, published by Nissan Motor Co., Ltd. For such electronically controlled automatic power transmission systems, elimination of shift shock which is otherwise caused by torque fluctuation upon a shifting up and a shifting down operation and also by a lag in apply and release timing of friction elements. In order to select an appropriate speed ratio, the automatic power transmission system employs a plurality of shifting control solenoids. For example, in the system disclosed in the aforementioned prior publications, first and second shift solenoids are active in response to a shift control system for an electronic transmission control unit for generating signal pressures for causing a shifting operation for selecting a speed ratio over four forward speed ratios and a single reverse ratio. A band brake provides a fourth speed servo apply pressure, a third speed servo relief pressure and a second speed servo apply pressure. When down-shifting from a fourth speed ratio to a second speed ratio is to take place, the fourth speed servo apply pressure and the third speed servo relief pressure are decreased. If the fourth speed servo apply pressure is released in advance of releasing of the third speed servo relief pressure, the position of the band brake is switched in a sequence ON-OFF-ON to cause shifting shock. In order to avoid this, a 4-2 sequence valve (first timing valve) is provided so as to assure that the releasing of the third speed servo relief pressure occurs in advance of releasing of the fourth speed servo apply pressure. In case that the 4-2 sequence valve is solely employed, shifting of the third speed ratio to the second speed ratio is performed in an order of 3rd-4th-2-nd/. In order to avoid this, a 4-2 relay valve (second timing valve) is provided. In this system, two timing control valves are required for adjusting shifting timing from the fourth speed ratio to the second speed ratio and thus causes an increase in the number of parts resulting in a high cost.
Another electronic control system for the automatic power transmission has been disclosed in Japanese Patent First Publication 55-60747. This publication proposes use of a multi-stage shift solenoid for eliminating shifting shock due to lag of engaging and releasing timing of the friction elements. This publication also may incur a high cost for a multi-stage shift valve. Furthermore, since a multi-stage shift valve is relatively bulky, the overall hydraulic control unit becomes bulky causing difficulty in installation on the automatic power transmission casing.